Certain phorbol esters, polar solvents and cytokines have been identified as in vitro inducers of human myeloid leukemic cell differentiation. However, the mechanisms by which these diverse groups of agents induce terminal maturation remain unclear. Furthermore, it is not clear whether agents that induce differentiation in vitro exert similar effects in vivo. The proposed studies will address these two related issues. Few insights are presently available regarding the intracellular signaling events activated by inducers of leukemic cell differentiation. However, recent studies suggest that the diverse classes of inducers similarly stimulate arachidonic acid metabolism during the induction of monocytic differentiation. The proposed studies will focus on the intracellular signaling events associated with phospholipid hydrolysis, appearance of the monocytic phenotype and changes in specific gene expression. The proposed studies will also continue to examine the therapeutic effects of differentiating agents in the treatment of myelodysplastic syndromes and acute myeloid leukemia. The induction of differentiation in vivo will be assessed by monitoring clonality of leukemic blasts and peripheral blood cells. The specific aims are: 1) to determine intracellular signaling mechanisms associated with induction of monocytic differentiation by agents that activate protein kinase C: 2) to study signal transduction pathways induced by polar solvent during myeloid differentiation; 3) to examine the effects of specific cytokines on induction of both intracellular signaling mechanisms and monocytic differentiation; and 4) to further study the effects of differentiating agents in the therapy of acute myeloid leukemia. These studies should provide fundamental insights regarding the intracellular signaling events induced by agents which overcome the block in leukemic cell differentiation. This work should also contribute to our understanding of the in vivo therapeutic effects of agents that induce differentiation in vitro.